Polymer pen lithography (PPL) was developed to address the challenge of depositing soft materials over large areas with nanoscale resolution. It is a cantilever-free technique that relies upon an array of elastomeric pyramidal pens, held in a plane by an elastomeric common substrate on a rigid support, to deposit nanoscale features in a massively parallel format across centimeter-scale areas. When each pen comes into contact with a surface to be patterned, it deforms in a manner that can be conceptually separated into (1) a tip deformation that increases the tip—sample contact area and (2) a support deformation that does not change the tip—sample contact area. While the increase of contact area upon deformation is useful as a means to vary feature size, it limits the minimum achievable feature size and sensitizes patterning to variations in pen array height. As a result, the minimum feature size achieved by PPL is larger than the smallest features written by its cantilever-based predecessor dip-pen nanolithography (DPN) by a factor of about 3. The dependence of feature size on tip—sample force further limits pattern uniformity because of (1) uncertainty in knowing the tip—sample height and (2) variation of tip height across the array. While both of these factors are reported to be under about 250 nm, there is nearly a 1:1 relationship between extension and feature size, so this effect can be quite significant when considering the desire to write large scale arrays of submicrometer features.
If the elastomeric pens are replaced with rigid silicon pens (while retaining the elastomeric backing film), force-independent patterning is possible. This technique, known as hard-tip, soft-spring lithography (HSL), offers an 8 μm range in extension over which the feature size does not change. The drawback to HSL is that making each pen array consumes a specialty 50 μm thick Si wafer, in contrast to PPL which utilizes pen arrays that can be molded nearly indefinitely from a single Si mold. Additionally, HSL pens are not transparent, which precludes their use for patterning with energy via optical methods. Other noteworthy attempts to improve resolution of PPL have relied on using pen arrays composed of other polymers or hard polymer pens on a soft elastomer support. These approaches reduce the feature size dependence on force by at most a quarter, but none have produced extension-independent patterning.
Thus, a need exists for a tip array and method of patterning that allows for printing with high resolution in an extension-independent manner using transparent pen arrays that are simple modifications of inexpensive PPL pen arrays.